The inept campaigns by McCain in 2008 and Romney in 2012 and the GOP's unwillingness to strongly confront Obama led to a widespread sense that the fight had gone out of the Republican establishment -- at least as to fighting the Democrats. The GOP establishment was eager though to mobilize and crush insurgent conservative candidates who challenged establishment favorites.
Cruz and his allies understand that if the GOP is to survive, it must energize its base by fighting the Democrats with as much energy and determination as is directed against the GOP. No longer can GOP members of Congress be well-paid time servers waiting and hoping that the political winds shift their way and deliver them into power. They need to fight for their declared political principles, and if they do not, the GOP may not survive.
Remarkably, even as the shutdown grinds on, the House GOP caucus seems to be firming up in their morale and willingness to do battle. They are taking a beating in the news media but getting hurrahs from the GOP base. The GOP members are still wearing Guccis and well-tailored suits (or pants suits), but they do seem to have a bit of fighter pilot swagger now.
The battle may break the GOP's way if the Obamacare rollout fizzles, Obama's poll numbers decline, Democratic constituencies start feeling and complaining about the effects of the shutdown, and polling shows that the shutdown costs them their chance at regaining the House and menaces Democratic control of the Senate.
There are signs that these things are beginning to happen. The GOP needs to continue the fight as long as victory is possible. If events and public opinion do not go the GOP's way, the well-trod path of ignominious Republican surrender will always be available.
An excellent, concise summary. This is what the mainstream media, and Democrats in general, fail to understand: the conservative insurgency is not a reaction to Obama per se. It began under Bush.